Fight as Shiki
Highlight the "Random Select" icon at the character selection screen and press Up(2), Down(2), Up, Down(2). Shiki will appear above the "Random Select" icon.
Fight as Zankuro
Highlight the "Random Select" icon at the character selection screen and press Down(2), Up(2), Down, Up(2) . Zankuro will appear below the "Random Select" icon.